1. Field of the Invention
The present invention relates generally to a method and apparatus for measuring spatial uniformity of a radiation beam from a pulsed or continuous radiation source and, more particularly, for simultaneously measuring the intensity of the radiation from the radiation source at a number of locations in a cross sectional area of the radiation beam to provide spatial uniformity data in less time and with improved temporal accuracy to characterize and facilitate alignment, adjustment, and calibration of the radiation source.
2. Description of the Related Art
Radiation sources are well-known and have a wide range of uses, ranging from standard light sources to x-ray machines to solar simulators. Typically, it is desirable that such radiation sources produce radiation beams having known or adjustable intensities, spectrums, and crosssectional shapes and sizes to suit a particular use. For example, radiation sources are useful for producing radiation beams that are used for testing or causing a predictable reaction with numerous materials and systems that react to a given type of radiation in a known and desired manner. For example, solar or photovoltaic cells are designed and constructed to receive solar radiation and convert it into electrical energy. It is important that the solar simulator radiation source provide a radiation beam having identical, or only slightly varying, intensity at any point in the cross section of the beam, i.e., spatial uniformity, so as to obtain acceptable test results and equal reaction rates across the tested material.
In particular, solar simulator-type radiation sources have .become increasingly important and are used to produce radiation beams with characteristics, such as intensity and spectrum, that simulate radiation that would be received from the sun at various geographic, atmospheric, or orbiting locations. In this way, solar simulators can be used to imitate actual field conditions, which is useful for testing photovoltaic conversion efficiencies of solar cells/arrays, resistances to solar radiation of various materials including sun screen compounds, numerous biological and. medical interactions with solar radiation, and other material or system properties. As, discussed above, for these tests to be accurate, i.e., give similar results at any point on the surface area being tested, it is necessary that the solar simulator produce a radiation beam with acceptable spatial uniformity. Certain American Society for Testing and Materials (ASTM) specifications dictate that the spatial uniformity for solar simulators be very high with a variance from a median intensity value of less than 10 percent for Class C, less than 5 percent for Class B, and even more restrictive, less than 2 percent for Class A. Without such high spatial uniformity, the test results can include errors that can go undetected thereby resulting in the test subject being rejected or even redesigned based on inaccurate test data. Therefore, an important and necessary step in using a radiation source, such as a solar simulator, is the alignment, adjustment, and calibration of the source to establish spatial uniformity, and, of course, it is desirable that this step be accomplished accurately and inexpensively.
A currently accepted method of checking spatial uniformity on a test surface, such as a photovoltaic array, involves placing a single radiation or photo detector at a first location on the test surface and measuring the intensity of a radiation beam produced by a radiation source. The detector is then moved to a number of other locations on the test surface, and the intensities of additional radiation beams from the radiation source are measured. A median intensity is calculated, and variance from this median intensity is determined at each measuring location on the test surface. If the results of the measured intensities and calculated variances indicate an unacceptable intensity variance in the radiation beam, the radiation source is adjusted in an attempt to better align the source to achieve an acceptable spatial uniformity. Each of these steps is then repeated until an acceptable spatial uniformity is achieved. As can be understood, this can be a tedious and time consuming process, especially with larger test surfaces, such as typical solar cell array modules, that require numerous measurements to provide an accurate representation of intensities across the entire surface area.
This procedure is used for aligning both continuous and pulse radiation sources with the assumption that temporal variation of the radiation beams produced by the source is negligible or in other words, that each beam produced is identical. In the case of a pulse source, each pulse is assumed to be equivalent in intensity and the intensity of the radiation beam is typically calculated by integrating or summing the intensity values over the entire length of the pulse, i.e., without making discrete measurements during transmission of the pulse beam. Additionally, the cost and difficulty of aligning/calibrating a radiation source are often further increased because making adjustments to the radiation source may be a complicated process itself that requires an operator to make simultaneous adjustments of several interrelated components to try to properly align the source.
For examples of various single-detector, radiation measuring devices, see U.S. Pat. No. 5,3274210 issued to Okui et al., U.S. Pat. No. 5,548,398 issued to Gaboury, and U.S. Pat. No. 4,218,139 issued to Sheffield.
Some efforts have been made to develop devices that can automate the movement or scanning of the single radiation detector across the radiation beam and that, at least potentially, can reduce error caused by the human placement and movement of the single radiation detector. For example, U.S. Pat. No. 3,867,036 issued to Detwiler et al. discloses a limit display circuit that includes a device for sequentially sampling or measuring intensities of a radiation beam by using a control motor to move a single photocell sensor across a radiation beam. However, use of this device for aligning and adjusting a radiation source is limited by the size, shape, and movement capabilities of the control motor, which itself has to be carefully calibrated and designed to control accuracy and can cause errors by introducing moving components into the testing device. As illustrated, the device is likely only useful for relatively small beams having linear cross-sectional shapes.
Consequently, there remains a need for devices and methods that will reduce the time that is required to measure the spatial uniformity of a radiation beam produced by a radiation source, such as a solar simulator, to facilitate quick, accurate, and inexpensive adjustment, alignment, and/or calibration of the radiation source.
Accordingly, it is a general object of the present invention to provide a method and apparatus for use in characterizing, aligning, adjusting, and/or calibrating a radiation source with improved accuracy and speed.
It is a related object of the present invention to provide a more effective method and apparatus for measuring the spatial uniformity of the intensity of a beam(s) from a radiation source(s).
It is a specific object of the present invention to provide a method and apparatus for use in aligning, adjusting, and/or calibrating pulse and continuous radiation sources that minimizes possible testing errors due to temporal variances of the radiation produced by the source.
It is another specific object of the present invention to provide a method and apparatus for determining spatial uniformity of a pulsed radiation source based on a single pulse.
It is another specific object of the present invention to provide a method and apparatus for determining spatial uniformity of a radiation source that does not require movement of a radiation detector(s).
Additional objects, advantages, and novel features of the invention are set forth in part in the description that follows and will become apparent to those skilled in the art upon examination of the following description and figures or may be learned by practicing the invention. Further, the objects and the advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, a measuring apparatus is provided for measuring, based on a single sampling time and/or a single pulse, the spatial uniformity of the radiation intensity in a beam produced by a radiation source. The measuring apparatus includes a detector array with a plurality of detectors that is positioned within the radiation beam such that the detectors concurrently receive portions of the radiation beam as it strikes the detector array. As the radiation source transmits a continuous beam or a pulsed beam, the, detectors operate to concurrently generate electrical signals proportional to the intensities of the portions of the radiation beam that impinge on the detectors. The measuring apparatus further includes a signal processor circuit connected to the detectors for concurrently sampling the detectors at a specific sampling time and then processing received electrical signals from the detectors to determine the spatial uniformity of the radiation beam.
A central processing unit (CPU) with memory and logic is provided in the signal processor circuit for determining radiation intensities of the radiation beam at each detector location in the detector array and for determining a median intensity value for the radiation beam, specifically, for the cross section of the radiation beam striking the detector array. The CPU then compares the determined radiation intensities to the median intensity value to determine relative intensity values of the radiation. These relative intensity values can then be displayed to an operator of the measuring apparatus, on an included data display device, relative or linked to the detector location and in a numerical manner (for example, xe2x80x9c100%xe2x80x9d if the relative intensity matches the median intensity value or xe2x80x9c97%xe2x80x9d or xe2x80x9c105%xe2x80x9d if it varies from the median intensity value) and/or in a pictorial manner (for example, a multicolored shape similar to that of the radiation beam cross section with certain colors indicating intensities matching the median intensity value and with certain other colors indicating lower and higher intensities). In this manner, the measuring. apparatus can be used by an operator to quickly determine spatial uniformity and make alignment and calibration steps on the radiation source based on the information shown on the data display device.
In one embodiment, a relatively large number of detectors, i.e., sixty-four, are included in the detector array to provide detailed sampling of the radiation beam. The detectors are positioned on a planar mounting plate in a pattern that forms a radiation receiving area having the approximate size and shape of the radiation beam cross section. The detectors in the radiation receiving area concurrently provide intensity information at substantially any point in the cross section of the radiation beam at the specific sampling time. In an alternate embodiment, the detectors are positioned to face varying directions to receive radiation from different directions and/or more than one radiation source to allow an operator, for example, to quickly identify radiation intensities on any number of surfaces and to then adjust the radiation source(s) to achieve desired illumination effects, such as photographic, cinematic, or other lighting effects.
To provide concurrent sampling of the detectors, the signal processor circuit in one embodiment includes a sample and hold circuit for each of the detectors. The sample and hold circuits are electrically connected to the detectors via an amplifier and are configured to receive the electrical signals from the detectors and to store the signals. A switching device, such as a multiplexer, is included in the signal processor circuit for selectively and sequentially switching between one or more of the sample and hold circuits to read or receive the signals and transmit the electrical signals on to an analog to digital (A/D) converter. The signals are then sequentially transmtitted to the CPU from the A/D converter in digital form. The CPU then processes the signals, as discussed above, while maintaining the link:between the digital signal and the location of the detector on the detector array to facilitate later display of relative intensity values relative to the detector locations. In an alternate embodiment, the signal processor circuit includes an A/D converter for each detector/amplifier combination, with each amplifier output fed to an A/D converter. Digital information from each A/D converter is fed in serial or parallel to the CPU. In both of the above embodiments, a timing circuit can be electrically connected to one or more detectors and the CPU to control tiring of radiation sampling by measuring the intensity of the radiation impinging on the detector(s) and providing a signal to begin concurrent sampling of the detectors when the radiation intensity reaches or falls below a preset or selectable intensity level.
To further achieve the foregoing and other objects, the present invention further comprises a method of measuring the spatial uniformity of the intensity of a radiation beam produced by a radiation source. The method includes positioning a detector array with a plurality of detectors transverse to an axis of the radiation beam, electrically connecting a signal processor circuit to each detector, receiving with the detectors portions of the radiation beam, concurrently transmitting with the detectors electrical signals representative of the intensities of the radiation impinging on the detectors to a signal processor circuit, and processing with a CPU in the signal processor circuit the electrical signals to determine the energy intensities of the radiation at each detector location, and therefore, determine the spatial uniformity of the radiation beam, based on a single sampling time and/or a single pulse from the radiation source.